Developing agent, method for manufacturing the same, and image forming apparatus

Abstract

A multi-layered toner comprises a core containing a first binder resin, and a covering layer containing a second binder resin having an acid value differing from that of the first binder resin. A magnetic powder is contained in at least one of the core and the covering layer.

What is claimed is:  
     
         1 . A developing agent including a multi-layered magnetic toner particle, said multilayered magnetic toner particle comprising: 
 a toner core containing a first binder resin having a first acid value,    a resin covering layer formed on said toner core and containing a second binder resin having a second acid value differing from said first acid value, and    a magnetic powder contained in at least one of said toner core and said resin covering layer.    
     
     
         2 . The developing agent according to  claim 1 , wherein said first acid value is not lower than 10.  
     
     
         3 . The developing agent according to  claim 2 , wherein said first acid value falls within a range of between 10 and 80.  
     
     
         4 . The developing agent according to  claim 1 , wherein said second acid value falls within a range of between 0 and 5.  
     
     
         5 . The developing agent according to  claim 1 , wherein said first acid value is not smaller than 10, and said second acid value falls within a range of between 0 and 5.  
     
     
         6 . The developing agent according to  claim 1 , wherein the difference between said first acid value and said second acid value falls within a range of between 5 and 80.  
     
     
         7 . The developing agent according to  claim 1 , wherein said toner core further contains a wax.  
     
     
         8 . The developing agent according to  claim 1 , wherein said toner core is formed by one process selected from the group consisting of a polymerization process and a pulverizing process.  
     
     
         9 . The developing agent according to  claim 1 , wherein said resin covering layer is formed by one process selected from the group consisting of a polymerization process and a dry process.  
     
     
         10 . The developing agent according to  claim 8 , wherein said dry process is selected from the group consisting of a mechanical process, a thermal process, a mechanochemical process and a complex process involving at least two of these dry processes.  
     
     
         11 . The developing agent according to  claim 8 , wherein said dry process is selected from the consisting of a mechanofusion process, a hybridization process and a surfusing process.  
     
     
         12 . The developing agent according to  claim 1 , wherein said magnetic powder is covered in advance with resin.  
     
     
         13 . The developing agent according to  claim 11 , wherein said resin is equal to said first binder resin.  
     
     
         14 . The developing agent according to  claim 1 , wherein said magnetic powder is attached to the surface of a core body containing mainly a binder resin.  
     
     
         15 . The developing agent according to  claim 1 , wherein said magnetic powder is dispersed in said toner core.  
     
     
         16 . The developing agent according to  claim 1 , wherein said resin covering layer further contains a magnetic powder covered in advance with resin.  
     
     
         17 . The developing agent according to  claim 1 , wherein said magnetic powder is contained in an amount of 30 to 70% by weight based on the total weight of the toner particles.  
     
     
         18 . A method of manufacturing a developing agent, comprising: 
 forming a resin covering layer on the surface of a toner core containing a first binder resin having a first acid value, said resin covering layer containing a second binder resin having a second acid value differing from said first acid value to obtain multi-layered magnetic toner particles; and    adding a magnetic powder in at least one of the first binder resin and the second binder resin.    
     
     
         19 . The method of manufacturing a developing agent according to  claim 18 , wherein said toner core is formed by one process selected from the group consisting of a polymerization process and a pulverizing process.  
     
     
         20 . The method of manufacturing a developing agent according to  claim 18 , wherein said resin covering layer is formed by one process selected from the group consisting of a polymerization process and a dry process.  
     
     
         21 . The method of manufacturing a developing agent according to  claim 20 , wherein said dry process is selected from the group consisting of a mechanical process, a thermal process, a mechanochemical process and a complex process involving at least two of these dry processes.  
     
     
         22 . The method of manufacturing a developing agent according to  claim 20 , wherein said dry process is selected from the consisting of a mechanofusion process, a hybridization process and a surfusing process.  
     
     
         23 . The method of manufacturing a developing agent according to  claim 18 , wherein said magnetic powder is covered in advance with resin.  
     
     
         24 . The method of manufacturing a developing agent according to  claim 23 , wherein said resin is equal to said first binder resin.  
     
     
         25 . An image forming apparatus, comprising: 
 an image carrier;    a developing mechanism including a developing device arranged to face said image carrier and serving to develop an electrostatic latent image formed on the surface of said image carrier to form a developing agent image;    a transfer mechanism for transferring said developing agent image onto a transfer material; and    a fixing device for fixing the developing agent image transferred to said transfer material;    wherein said developing agent is housed in said developing device, includes multi-layered magnetic toner particles, said multi-layered toner particles comprising a toner core containing a first binder resin having a first acid value, a resin covering layer formed on said toner core and containing a second binder resin having a second acid value differing from said first acid value, and a magnetic powder contained in at least one of said toner core and said resin covering layer.